Starting in 1966 the COOPERATIVE STUDY ON THE TREATMENT OF HYPERTENSION is a continuing, prospective, carefully controlled, "blinded", long-term investigation of the effects of treating a large and statistically significant number of mild-moderate hypertensive patients with reserpine-chlorthiazide or with a placebo. Both treatment groups are carefully followed by physical examination, blood pressure recording, chest X-rays, electrocardiograms and other appropriate clinical and laboratory tests. The nature and number of all "morbid events" are carefully documented by impartial referee. The present investigation of the efficacy of hypotension drugs in preventing serious medical complications in patients with mild-moderate hypertension is both very significant and very important because of: 1) The prevalence in the general population of hypertension, the complications of hypertension and the complications of atherosclerosis (which appear to be associated with hypertension). 2) The public health importance of preventing or delaying the appearance of these serious complications. 3) The availability of relatively effective, safe and inexpensive oral hypotensive drugs. 4) The lack of any definitive study establishing the efficacy of treatment in mild-moderate hypertension - i.e. the lack of a carefully controlled, prospective, "blinded", long- term study of a statistically significant number of patients.